The dynamics of ground vehicles are largely determined by forces between its tires and the road. Knowledge of these forces is beneficial for most active vehicle control systems. Several strategies have been developed for estimating tire forces in three axes (e.g., longitudinal, lateral, and vertical). For example, it has been reported that the measurement of deformation of a magnetized tire may be used to determine estimates of tire longitudinal and lateral forces. This strategy may have several shortcomings including the need for transmission of data from a rotating tire, a requirement of an excessive amount of data processing, and a lack of robustness due to a need for using tire stiffness characteristics in relating tire forces to measured deformation. The tire stiffness characteristics may further be complicated by variations in pressure, rotation speed, and temperature.
One strategy for estimating tire vertical forces using inertial sensors is disclosed in U.S. Pat. No. 6,593,849 issued Jul. 15, 2003 to Chubb, et al. A two wheel lift off condition (i.e., when the vertical force on two tires of the same side of vehicle is zero) may be detected thereby indicating a danger of vehicle rollover. The strategy disclosed by the '849 patent, however, is limited to a calculation of vertical tire forces and therefore does not provide a determination of longitudinal and lateral forces.
In U.S. Pat. No. 6,549,842 issued Apr. 15, 2003 to Hac, et al., a strategy for estimating a surface coefficient of adhesion for each wheel is disclosed, which involves a determination of tire dynamic forces. This strategy, however, does not account for the effects of vehicle body pitch and roll dynamics or wheel vertical dynamics, which may limit the accuracy of estimates under dynamic conditions (e.g., driving on a rough road). Moreover, tire forces are calculated from models that depend on several variables (e.g., tire longitudinal slip, tire sideslip angle, and surface coefficient of adhesion), which cannot be measured directly (at a reasonable cost) and are difficult to estimate with precision.
Therefore, it would be desirable to provide a strategy for operating a vehicle control system based on determined dynamic tire forces that overcomes the aforementioned and other disadvantages.